The present invention is directed to a method for the control of the oven dry density of a moving sheet material and more specifically to the density of ceiling tile.
In the manufacture of ceiling tile it is not possible to make on-line measurement of its moisture content. Thus, the final density after being cut and passed through dryers, known as the oven dry density, is normally estimated. Density is, of course, the total basis weight divided by the thickness of caliper of the ceiling tile.
The two parameters for control of ceiling tile are the stuff gate which is in essence the stock flow of raw material to the headbox and the press which is the force used to control the thickness. In the past, manual control of these two parameters was used to produce the final desired dry density. Specifically, this was accomplished every 30 minutes by the machine tender who took a caliper or thickness reading automatically or with a hand gauge and a standard size sample. The wet weight was measured and it was then dried in a microwave oven and the dry weight measured. With this data, the machine tender calculated an estimated dry density which is an estimate of the product density after leaving the dryer. The estimated dry density was then compared to the desired or target dry density and either the stuff gate and/or the press parameters were controlled accordingly. With the foregoing manual control method, the final dry density and concommitant caliper or thickness varied excessively. For example, estimated dry density might vary by more than two pounds per cubic feet and caliper by more than 0.010 inches.